Monometallic rhodium-containing four-way conversion catalysts for gasoline engine emissions treatment systems

ABSTRACT

Catalyzed particulate filters comprise three-way conversion (TWC) catalytic material, which comprises rhodium as the only platinum group metal, that permeates walls of a particulate filter. Such catalyzed particulate filters may be located downstream of close-coupled three-way conversion (TWC) composites in an emission treatment system downstream of a gasoline direct injection engine for treatment of an exhaust stream comprising hydrocarbons, carbon monoxide, nitrogen oxides, and particulates.

TECHNICAL FIELD

This invention pertains generally to emission treatment systems havingcatalysts used to treat gaseous streams of gasoline engines containinghydrocarbons, carbon monoxide, and oxides of nitrogen in conjunctionwith particulates. More specifically, this invention is directed to afour-way converter (FWC), which comprises a three-way conversion (TWC)catalyst that is monometallic, comprising rhodium only, and aparticulate filter.

BACKGROUND

Particulate emissions for gasoline engines are being subject toregulations, including Euro 6 (2014) standards. Certain gasoline directinjection (GDI) engines have been developed whose operating regimesresult in the formation of fine particulates. Aftertreatment systems forgasoline engines need to achieve particulate matter standards. Incontrast to particulates generated by diesel lean burning engines, theparticulates generated by gasoline engines, such as GDI engines, tend tobe finer and in lesser quantities. This is due to the differentcombustion conditions of a diesel engine as compared to a gasolineengine. For example, gasoline engines run at a higher temperature thandiesel engines. Also, hydrocarbon components are different in theemissions of gasoline engines as compared to diesel engines.

Emissions of unburned hydrocarbons, carbon monoxide and nitrogen oxidecontaminants continue to be regulated. Catalytic converters containing athree-way conversion (TWC) catalyst are accordingly located in theexhaust gas line of internal combustion engines. Such catalysts promotethe oxidation by oxygen in the exhaust gas stream of unburnedhydrocarbons and carbon monoxide as well as the reduction of nitrogenoxides to nitrogen.

A catalyzed particulate trap comprising a TWC catalyst coated onto orwithin a particulate trap is provided in U.S. Pat. No. 8,173,087 (Wei).A gasoline engine emissions treatment system having particulate filtersis also provided in U.S. Pat. No. 8,815,189 (Arnold).

Emissions technologies are limited by backpressure and volumeconstraints of exhaust systems. That is, within defined backpressuresand volumes, any new technologies should have minimal to no impact oneither.

There is a continuing need to provide a catalyzed filter that providessufficient and cost-effective TWC in conjunction with an efficientfilter without unduly increasing backpressure so that regulated HC, NOx,and CO conversions can be achieved while meeting particulate matteremissions.

SUMMARY

Provided are exhaust systems and components suitable for use inconjunction with gasoline engines to capture particulates in addition totreating gaseous emissions such as hydrocarbons, nitrogen oxides, andcarbon monoxides. Of interest is providing a particulate filter forgasoline engines (GPFs or PFGs) for use downstream of a traditionalthree-way conversion (TWC) so that the combination provides full TWCfunctionality.

In a first aspect, provided is an emission treatment system downstreamof a gasoline direct injection engine for treatment of an exhaust streamcomprising hydrocarbons, carbon monoxide, nitrogen oxides, andparticulates, the emission treatment system comprising:

-   -   a close-coupled three-way conversion (TWC) composite comprising        a first TWC catalytic material on a flow-through substrate; and    -   an catalyzed particulate filter located downstream of the        close-coupled TWC composite, the catalyzed particulate filter        comprising a second TWC catalytic material that permeates walls        of a particulate filter;    -   wherein the second TWC catalytic material comprises rhodium as        the only platinum group metal.

The particulate filter may comprise a mean pore diameter in the range ofabout 13 to about 25 μm. The particulate filter may comprise a wallthickness in the range of about 6 mils (152 μm) to about 14 mils (356μm) and an uncoated porosity in the range of 55 to 70%. The catalyzedparticulate filter may have a coated porosity that is less than anuncoated porosity of the particulate filter. In a detailed embodiment,there is no layering of catalytic material on the surface of the wallsof the particulate filter except optionally in areas of overlappedwashcoat. In another detailed embodiment, the coated porosity islinearly proportional to a washcoat loading of the TWC catalyticmaterial. The coated porosity may be between 75 and 98% of the uncoatedporosity. The coated porosity may be between 80 and 95% of the uncoatedporosity. The coated porosity may be between 80 and less than 93% of theuncoated porosity. A coated backpressure of the catalyzed particulatefilter is generally non-detrimental to performance of the engine. Thesecond TWC catalytic material may comprise a d90 average particlediameter in the range of about 2.5 to about 8 μm. The second TWCcatalytic material may be formed from a single washcoat composition thatpermeates an inlet side, an outlet side, or both of the particulatefilter.

A first single washcoat layer may present on the inlet side along up toabout 0-100% of the axial length of the particulate filter from theupstream end and a second single washcoat layer may be present on theoutlet side along up to about 0-100% of the axial length of theparticulate filter from the downstream end, wherein at least one of thefirst and single washcoat layers is present in an amount of >0%.

A first single washcoat layer may be present on the inlet side along upto about 50-100% of the axial length of the particulate filter from theupstream end and a second single washcoat layer may be present on theoutlet side along up to about 50-100% of the axial length of theparticulate filter from the downstream end. The first single washcoatlayer may be present on the inlet side along up to about 50-55% of theaxial length of the particulate filter from the upstream end and thesecond single washcoat layer may be present on the outlet side along upto about 50-55% of the axial length of the particulate filter from thedownstream end.

A single washcoat layer may be present on the inlet side along up toabout 100% of the axial length of the particulate filter from theupstream end and there is not a washcoat layer on the outlet side.

A single washcoat layer may be present on the outlet side along up toabout 100% of the axial length of the particulate filter from thedownstream end and there is not a washcoat layer on the inlet side.

The second TWC catalytic material may be present in an amount in therange of about 0.17 to about 5 g/in³ (about 10 to about 300 g/L).

The second TWC catalytic material may consist essentially of rhodium,ceria or a ceria composite, and alumina.

Another aspect provides a catalyzed particulate filter located in anemission treatment system downstream of a gasoline direct injectionengine for treatment of an exhaust stream comprising hydrocarbons,carbon monoxide, nitrogen oxides, and particulates and downstream of athree-way conversion (TWC) composite comprising a first TWC catalyticmaterial on a flow-though substrate, the catalyzed particulate filtercomprising:

-   -   a particulate filter comprising a wall thickness in the range of        about 6 mils (152 μm) to about 14 mils (356 μm) and a porosity        in the range of 55 to 70%; and    -   a second three-way conversion (TWC) catalytic material in an        amount in the range of about 0.17 to about 5 g/in³ (10 to 300        g/L), the second TWC catalytic material comprising rhodium as        the only platinum group metal;    -   wherein the catalyzed particulate filter has a coated porosity        that is less than an uncoated porosity of the particulate filter        and a coated backpressure that is substantially the same as an        uncoated backpressure of the particulate filter.

The wall thickness may be about 8 mils; the amount of the secondthree-way conversion (TWC) catalytic material may be in the range ofabout 0.17 to about 1.5 g/in³ (10 to 90 g/L), the second TWC catalyticmaterial comprising rhodium as the only platinum group metal; and theparticulate filter may comprise a mean pore size distribution in therange of about 13 to about 25 μm. Another aspect is a method of treatingan exhaust gas comprising hydrocarbons, carbon monoxide, nitrogenoxides, and particulates, the method comprising: obtaining a catalyzedparticulate filter according any embodiment disclosed herein; andlocating the catalyzed particulate filter downstream of a gasolinedirect injection engine and a three-way conversion (TWC) compositecomprising a first TWC catalytic material on a flow-through substrate;wherein upon operating of the engine, exhaust gas from the gasolinedirect injection engine contacts the catalyzed particulate filter.

A further aspect is a method of making emission treatment system for agasoline direct injection engine, the method comprising: positioning athree-way conversion (TWC) composite comprising a first TWC catalyticmaterial on a flow-through substrate downstream of the gasoline directinjection engine; obtaining a catalyzed particulate filter comprising asecond three-way conversion (TWC) catalytic material permeating walls ofa particulate filter, the particulate filter comprising a wall thicknessin the range of about 6 mils (152 μm) to about 14 mils (356 μm) and aporosity in the range of 55 to 70% and the second TWC catalytic materialcomprising rhodium as the only platinum group metal; positioning thecatalyzed particulate filter downstream of the TWC composite.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1 is a schematic view showing an engine emission treatment systemaccording to a detailed embodiment;

FIG. 2 is a perspective view of a wall flow filter substrate;

FIG. 3 is a cut-away view of a section of a wall flow filter substrate;and

FIGS. 4-6 provide schematic views of FWC coating designs.

DETAILED DESCRIPTION

Provided are filters for gasoline direct injection engines (GDI) thatare designed to achieve high particle filtration efficiency andcost-effective gaseous emissions conversion. State of the art gasolinecatalytic after-treatment systems generally include two catalysts: afirst one close to the engine (e.g., CC: close-coupled position) and asecond one downstream of the first one farther along the exhaustafter-treatment system (e.g., UF: under floor position). Catalystsapplied in such CC+UF configuration bear different temperature stabilityand conversion efficiency requirements: the catalyst in CC position,being closer to the engine, requires a higher thermal resistance thanthe catalyst placed in UF position. Catalyst formulations for FWC hereinare designed to provide a cost-effective solution. It is subject of thisinvention a FWC formulation for UF position that only utilizes amonometallic platinum group metal, rhodium (Rh), as active preciousmetal, avoiding completely the use of palladium (Pd), thus providing theopportunity to significantly reduce costs.

Historically, TWC formulations used in the UF position comprise, asactive precious metal, both Pd and Rh. Pd is commonly used together withboth alumina and oxygen storage components to catalyze hydrocarbon (HC)oxidation and to activate the Ce3+/Ce4+ redox reaction respectively. Ithas been found that the amount of Pd used in TWC catalyst in UF positionis not sufficient to significantly reduce HC emissions and that theCe3+/Ce4+redox reaction can be effectively activated also by opportuneuse of only Rh.

The following definitions are used herein.

As used herein, the term “permeate” when used to describe the dispersionof the TWC catalyst into porous walls of a particulate filter means thatthe particular composition penetrates into at least a majority of thehollow regions within the wall thickness, and becomes deposited on theinternal surfaces throughout the thickness of the walls. In this mannerthe material becomes dispersed throughout the wall of the filter.

Porosity of the particulate filters is a percentage of the volume ofpores of the filter relative to the volume of the filter. One way tomeasure porosity is by mercury porosimetry. A filter may be sectioned,and porosity of each section is measured, and the results are averaged.For example, a filter can be sectioned into a front/inlet piece, amiddle piece, and a rear/outlet piece, the porosity of each piece can betaken, and the results can be averaged. An uncoated porosity is theporosity of the filter, which does not have any catalytic materialapplied to it. A coated porosity is the porosity of a catalyzed filter,which is the combination of catalytic material and a filter. Catalyzedparticulate filters can have a coated porosity that is less than anuncoated porosity of the particulate filter, which indicates that thewashcoat resides in the pores of the filter and not on the surface ofthe walls. Some methods used herein result in a coated porosity that islinearly proportional to a washcoat loading of the TWC catalyticmaterial because the material resides in the pores and not on the wallsof the filter. The coated porosity may be between 75 and 98%, or 80 and95%, or even 80 and 93% of the uncoated porosity.

Backpressure of the filters is a measure of resistance of flow throughthe filter, expressed in, for example, units of mbar. An uncoatedbackpressure is the backpressure of the filter, which does not have anycatalytic material applied to it. A coated backpressure is thebackpressure of a catalyzed filter, which is the combination ofcatalytic material and a filter. Catalyzed particulate filters can havea coated backpressure that is non-detrimental to the performance of theengine. A non-detrimental pressure drop means that the engine willperform generally the same (e.g., fuel consumption) in a wide range ofengine operational modes in the presence of a filter substrate that iseither in a coated or an uncoated state.

“FWC” refers to four-way conversion where in addition to three-wayconversion (TWC) functionality, which is defined next, there is afiltering function.

“TWC” refers to the function of three-way conversion where hydrocarbons,carbon monoxide, and nitrogen oxides are substantially simultaneouslyconverted. A gasoline engine typically operates under nearstoichiometric reaction conditions that oscillate or are pertubatedslightly between fuel rich and fuel lean air to fuel ratios (A/F ratios)(λ=1±˜0.01), at perturbation frequencies of 0.5 to 2 Hz. Use of“stoichiometric” herein refers to the conditions of a gasoline engine,accounting for the oscillations or pertubations of NF ratios nearstoichiometric. TWC catalysts include oxygen storage components (OSCs)such as ceria that have multi-valent states which allows oxygen to beheld and released under varying air to fuel ratios. Under richconditions when NOx is being reduced, the OSC provides a small amount ofoxygen to consume unreacted CO and HC. Likewise, under lean conditionswhen CO and HC are being oxidized, the OSC reacts with excess oxygenand/or NOx. As a result, even in the presence of an atmosphere thatoscillates between fuel rich and fuel lean air to fuel ratios, there isconversion of HC, CO, and NOx all at the same (or at essentially all thesame) time. Typically, a TWC catalyst comprises one or more platinumgroup metals such as palladium and/or rhodium and optionally platinum;an oxygen storage component; and optionally promoters and/orstabilizers. Under rich conditions, TWC catalysts can generate ammonia.

Reference to “full TWC functionality” means that HC and CO oxidation andNOx reduction can be achieved in accordance with requirements ofregulatory agencies and/or car manufacturers.

In this way, platinum group metal components such as platinum,palladium, and rhodium are provided to achieve HC, CO, and NOxconversions and sufficient oxygen storage components (OSC) are providedto achieve sufficient oxygen storage capacity to ensure proper HC, NOx,and CO conversion in an environment of varying NF (air-to-fuel) ratios.Sufficient oxygen storage capacity generally means that after a fulluseful life aging as defined by a car manufacturer, the catalyst canstore and release a minimum amount of oxygen. In one example, a usefuloxygen storage capacity can be 100 mg per liter of oxygen. For anotherexample, a sufficient oxygen storage capacity can be 200 mg per liter ofoxygen after 80 hours of exothermic aging at 1050° C. Sufficient oxygenstorage capacity is needed to ensure that on-board diagnostics (OBD)systems detect a functioning catalyst. In the absence of sufficientoxygen storage capacity, the OBD will trigger an alarm of anon-functioning catalyst. High oxygen storage capacity is more than thesufficient amount, which widens the operating window of the catalyst andpermits more flexibility in engine management to a car manufacturer.

Reference to oxygen storage component (OSC) refers to an entity that hasmulti-valence state and can actively react with oxidants such as oxygenor nitrogen oxides under oxidative conditions, or reacts with reductantssuch as carbon monoxide (CO) or hydrogen under reduction conditions.Examples of suitable oxygen storage components include ceria.Praseodymia can also be included as an OSC. Delivery of an OSC to thewashcoat layer can be achieved by the use of, for example, mixed oxides.For example, ceria can be delivered by a mixed oxide of cerium andzirconium, and/or a mixed oxide of cerium, zirconium, and neodymium. Forexample, praseodymia can be delivered by a mixed oxide of praseodymiumand zirconium, and/or a mixed oxide of praseodymium, cerium, lanthanum,yttrium, zirconium, and neodymium.

TWC catalysts that exhibit good activity and long life comprise one ormore platinum group metals (e.g., platinum, palladium, rhodium, rheniumand iridium) disposed on a high surface area, refractory metal oxidesupport, e.g., a high surface area alumina coating. The support iscarried on a suitable carrier or substrate such as a monolithic carriercomprising a refractory ceramic or metal honeycomb structure, orrefractory particles such as spheres or short, extruded segments of asuitable refractory material. The refractory metal oxide supports may bestabilized against thermal degradation by materials such as zirconia,titania, alkaline earth metal oxides such as baria, calcia or strontiaor, most usually, rare earth metal oxides, for example, ceria, lanthanaand mixtures of two or more rare earth metal oxides. For example, seeU.S. Pat. No. 4,171,288 (Keith). TWC catalysts can also be formulated toinclude an oxygen storage component.

Reference to a “support” in a catalyst washcoat layer refers to amaterial that receives precious metals, stabilizers, promoters, binders,and the like through association, dispersion, impregnation, or othersuitable methods. Examples of supports include, but are not limited to,high surface area refractory metal oxides and composites containingoxygen storage components. High surface refractory metal oxide supportsrefer to support particles having pores larger than 20 Å and a wide poredistribution. High surface area refractory metal oxide supports, e.g.,alumina support materials, also referred to as “gamma alumina” or“activated alumina,” typically exhibit a

BET surface area in excess of 60 square meters per gram (“m²/g”), oftenup to about 200 m²/g or higher. Such activated alumina is usually amixture of the gamma and delta phases of alumina, but may also containsubstantial amounts of eta, kappa and theta alumina phases. Refractorymetal oxides other than activated alumina can be used as a support forat least some of the catalytic components in a given catalyst. Forexample, bulk ceria, zirconia, alpha alumina and other materials areknown for such use. Although many of these materials suffer from thedisadvantage of having a considerably lower BET surface area thanactivated alumina, that disadvantage tends to be offset by a greaterdurability of the resulting catalyst. “BET surface area” has its usualmeaning of referring to the Brunauer, Emmett, Teller method fordetermining surface area by N₂ adsorption.

One or more embodiments include a high surface area refractory metaloxide support comprising an activated compound selected from the groupconsisting of alumina, alumina-zirconia, alumina-ceria-zirconia,lanthana-alumina, lanthana-zirconia-alumina, baria-alumina, barialanthana-alumina, baria lanthana-neodymia alumina, and alumina-ceria.Examples of composites containing oxygen storage components include, butare not limited to, ceria-zirconia and ceria-zirconia-lanthana.Reference to a “ceria-zirconia composite” means a composite comprisingceria and zirconia, without specifying the amount of either component.Suitable ceria-zirconia composites include, but are not limited to,composites having, for example, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%,45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90% or even 95% of ceriacontent. Certain embodiments provide that the support comprises bulkceria having a nominal ceria content of 100% (i. e., >99% purity).

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

Turning to FIG. 1, an emissions treatment system 3 comprises a gasolineengine 5 that conveys exhaust through line 7 to a first TWC catalyst 9,which is in a close-coupled (CC) position. A downstream TWC-coatedparticulate filter (FWC) 13, which receives the exhaust stream throughline 11, is in an underfloor (UF) position. Line 15 can lead to furthertreatment components and/or to the tail pipe and out of the system. TheTWC-coated particulate filter 13 contains a TWC catalyst loading that isdesigned to work in conjunction with the CC TWC catalyst in order tocollectively provide full TWC functionality, thereby meeting emissionrequirements.

Particulate Filter

Reference to particulate filter means a substrate so sized andconfigured to trap particulates generated by the combustion reactions inthe direct injection gasoline engine. Trapping of particulates canoccur, for example, by use of a particulate (or soot) filter, by use ofa flow-through substrate having an internal tortuous path such that achange in direction of flow of the particu lates causes them to drop outof the exhaust stream, by use of a metallic substrate, such as acorrugated metal carrier, or by other methods known to those skilled inthe art. Other filtration devices may be suitable, such as a pipe with aroughened surface that can knock particles out of the exhaust stream. Apipe with a bend may also be suitable.

With reference to filters, FIG. 2 depicts a perspective view of anexemplary wall flow filter substrate suitable for a particulate filter.Wall flow substrates useful for supporting the TWC or oxidation catalystcompositions have a plurality of fine, substantially parallel gas flowpassages extending along the longitudinal axis (or axial length) of thesubstrate. Typically, each passage is blocked at one end of thesubstrate body, with alternate passages blocked at opposite endfaces.Such monolithic carriers may contain up to about 300 flow passages (or“cells”) per square inch of cross section, although far fewer may beused. For example, the carrier may have from about 7 to 300, moreusually from about 200 to 300, cells per square inch (“cpsi”). The cellscan have cross sections that are rectangular, square, circular, oval,triangular, hexagonal, or are of other polygonal shapes. Wall flowsubstrates for FWC typically have a wall thickness between 6-14 mils or152-356 μm. Axial zoning may be desirable such that a coating isprovided along an axial length of the filter. On the inlet side, asmeasured from the upstream end 54, a coating may extend up to 50% of theaxial length (e. g., 1 to 49.9%, or 10 to 45%), 50 to 75% of the axiallength, or even 100% of the axial length. On the outlet side, asmeasured from the downstream end 56, a coating may extend up to 50% ofthe axial length (e. g., 1 to 49.9%, or 10 to 45%), 50 to 75% of theaxial length, or even 100% of the axial length.

FIGS. 2 and 3 illustrate a wall flow filter substrate 50 that has aplurality of passages 52. The passages are tubularly enclosed by theinternal walls 53 of the filter substrate. The substrate has an inlet orupstream end 54 and an outlet or downstream end 56. Alternate passagesare plugged at the inlet end with inlet plugs 58 and at the outlet endwith outlet plugs 60 to form opposing checkerboard patterns at the inlet54 and outlet 56. A gas stream 62 enters at upstream end 54 through theunplugged channel inlet 64, is stopped by outlet plug 60 and diffusesthrough channel walls 53 (which are porous) to the outlet side 66. Acoating on the inlet side of the filter means that the coating resideson or within the walls 53 such that the gas stream 62 contacts the inletcoating first. A coating on the outlet side of the filter means that thecoating resides on or within the walls 53 such that the gas stream 62contacts the outlet coating after the inlet coating. The gas cannot passback to the inlet side of walls because of inlet plugs 58.

In FIG. 4, a first washcoat 102 is provided 50-55% of the length of theinlet side and a second washcoat 104 is provided 50-55% of the length ofthe outlet side. The embodiment of FIG. 4 may be suitable for highwashcoat loadings, where overall washcoat loading is 1.5 g/in³, forexample: 1.5-3 g/in³, or even 2.5 g/in³. In FIG. 5, a single washcoat102 is provided up to 100% of the length of the inlet side, whichincludes >0% to 100% and all values in between, and there is not awashcoat provided on the outlet side. The embodiment of FIG. 5 may besuitable for low washcoat loadings, where overall washcoat loading is<1.5 g/in³, for example: 0.25-<1.5 g/in³, or even 0.5-1.0 g/in³. In FIG.6, a single washcoat 104 is provided up to 100% of the length of theoutlet side, which includes >0% to 100% and all values in between, andthere is not a washcoat provided on the inlet side. The embodiment ofFIG. 6 may also be suitable for low washcoat loadings, where overallwashcoat loading is <1.5 g/in³, for example: 0.25-<1.5 g/in³, or even0.5-1.0 g/in³. In FIGS. 4-6, the washcoats may be located on and/orpermeate the walls. In a preferred embodiment, the washcoat permeatesthe walls and is not located on the walls.

Wall flow filter substrates can be composed of ceramic-like materialssuch as cordierite, alumina, silicon carbide, aluminum titanate,mullite, or of refractory metal. Wall flow substrates may also be formedof ceramic fiber composite materials. Specific wall flow substrates areformed from cordierite, silicon carbide, and aluminum titanate. Suchmaterials are able to withstand the environment, particularly hightemperatures, encountered in treating the exhaust streams. Wall flowsubstrates for use in the inventive system can include thin porouswalled honeycombs (monoliths) through which the fluid stream passeswithout causing too great an increase in back pressure or pressureacross the article. Ceramic wall flow substrates used in the system canbe formed of a material having a porosity (also referred to as uncoatedporosity) of at least 40% (e.g., from 40 to 70% or even 55 to 70%).Useful wall flow substrates can have an mean pore size of 10 or moremicrons, preferably 13 to 25 microns. When substrates with theseporosities and these mean pore sizes are coated with the techniquesdescribed below, adequate levels of TWC compositions can be loaded ontothe substrates to achieve excellent hydrocarbon, CO, and/or NOxconversion efficiency. These substrates are still able retain adequateexhaust flow characteristics, i.e., acceptable back pressures, despitethe catalyst loading.

The porous wall flow filter used in this invention is catalyzed in thatthe wall of the element has thereon or contained therein one or morecatalytic materials. Catalytic materials may be present on the inletside of the element wall alone, the outlet side alone, both the inletand outlet sides, or the wall itself may consist all, or in part, of thecatalytic material. This invention includes the use of one or morewashcoats of catalytic materials and combinations of one or morewash-coats of catalytic materials on the inlet and/or outlet walls ofthe element.

With reference to a metallic substrate, a useful substrate may becomposed of one or more metals or metal alloys. The metallic carriersmay be employed in various shapes such as corrugated sheet or monolithicform. Specific metallic supports include the heat resistant metals andmetal alloys such as titanium and stainless steel as well as otheralloys in which iron is a substantial or major component. Such alloysmay contain one or more of nickel, chromium and/or aluminum, and thetotal amount of these metals may advantageously comprise at least 15 wt% of the alloy, e.g., 10-25 wt % of chromium, 3-8 wt % of aluminum andup to 20 wt % of nickel.

The alloys may also contain small or trace amounts of one or more othermetals such as manganese, copper, vanadium, titanium and the like. Thesurface of the metal carriers may be oxidized at high temperatures,e.g., 1000° C. and higher, to improve the resistance to corrosion of thealloys by forming an oxide layer on the surfaces of the carriers. Suchhigh temperature-induced oxidation may enhance adherence of a catalyticmaterial to the carrier.

Coating Wall Flow Filters

To coat wall flow filters with the TWC or oxidation catalyst compositionusing a traditional technique, a mixture of ingredients is preparedusing metal salts, which are usually a mixture of organic and inorganicsalts, to form a catalyst slurry. Such slurries may typically have adynamic viscosity of 14 to 400 mPa·s at 20° C. or greater with a solidscontent in the range of 25% to 0% of solids. Substrates are immersedvertically in a portion of the catalyst slurry such that the top of thesubstrate is located just above the surface of the slurry. In thismanner, slurry contacts the inlet face of each honeycomb wall, but isprevented from contacting the outlet face of each wall. The sample isleft in the slurry for about 30-60 seconds. The filter is removed fromthe slurry, and excess slurry is removed from the wall flow filter firstby allowing it to drain from the channels, then by blowing withcompressed air (against the direction of slurry penetration). By usingthis traditional technique, the catalyst slurry permeates the walls ofthe filter, yet the pores are not occluded to the extent that undue backpressure will build up in the finished filter. By using this traditionaltechnique, the coated porosity of the filter is expected to besubstantially the same as its uncoated porosity. The coated filters aredried typically at about 100° C. and calclined at a higher temperature(e.g., 300 to 450° C. and up to 590° C.). After calcining, the catalystloading can be determined through calculation of the coated and uncoatedweights of the filter. As will be apparent to those of skill in the art,the catalyst, loading can be modified by altering the solids content ofthe coating slurry. Alternatively, repeated immersions of the filter inthe coating slurry can be conducted, followed by removal of the excessslurry as described above.

To coat wall flow filters with the TWC or oxidation catalyst compositionusing an improved technique, a mixture of ingredients is prepared usinginorganic metal salts only to form a catalyst slurry that has a lowviscosity relative to the traditional technique. Such slurries maytypically have a dynamic viscosity in the range of ˜5 to less than 40mPa·s at 20° C., or ˜5 to less than 30 mPa·s, with a solids content inthe range of 25% to 0%. The slurry viscosity is much lower than thetraditional technique by at least 50% or even more such as 75-90%. Thenumber of processing steps is reduced compared to the traditionaltechnique. Substrates are immersed vertically in a portion of thecatalyst slurry for the length of the substrate equal to the targetedlength of the coat to be applied. In this manner, slurry contacts theinlet face of each honey-comb wall and penetrates the wall completelyfor the length of immersion. The sample is left in the slurry for about1-6 seconds. The filter is removed from the slurry, and excess slurry isremoved from the wall flow filter first by allowing it to drain from thechannels, then by blowing with compressed air (against the direction ofslurry penetration). By using this improved technique, the catalystslurry permeates the walls of the filter, yet the pores are not occludedto the extent that undue back pressure will build up in the finishedfilter. By using this improved technique, the coated porosity of thefilter is lower than its uncoated porosity in that the washcoat residesprimarily to completely in the pores of the filter and not on thesurface of the walls. Furthermore, relative to the traditionaltechnique, improved homogeneity of slurry distribution along the coatedlength is achieved due to more efficient penetration of the low viscousslurry into the walls. Finally, by using such technique and as a resultof the improved slurry penetration into the wall and homogeneity, lowerback pressure increase is achieved relative of the finished filterrelative to the traditional technique described above. The coatedfilters are dried typically at about 100° C. and calcined at a highertemperature (e.g., 300 to 450° C. and up to 590° C.). After calcining,the catalyst loading can be determined through calculation of the coatedand uncoated weights of the filter. As will be apparent to those ofskill in the art, the catalyst, loading can be modified by altering thesolids content of the coating slurry. Alternatively, repeated immersionsof the filter in the coating slurry can be conducted, followed byremoval of the excess slurry as described above.

Preparation of Catalyst Composite Washcoats

The catalyst composites may be formed in a single layer or multiplelayers. In some instances, it may be suitable to prepare one slurry ofcatalytic material and use this slurry to form multiple layers on thecarrier. The composites can readily prepared by processes well known inthe prior art. A representative process is set forth below. As usedherein, the term “washcoat” has its usual meaning in the art of a thin,adherent coating of a catalytic or other material applied to a substratecarrier material, such as a honeycomb-type carrier member, which issufficiently porous to permit the passage there through of the gasstream being treated. A “washcoat layer,” therefore, is defined as acoating that is comprised of support particles. A “catalyzed washcoatlayer” is a coating comprised of support particles impregnated withcatalytic components.

The catalyst composite can be readily prepared in layers on a carrier.For a first layer of a specific washcoat, finely divided particles of ahigh surface area refractory metal oxide such as gamma alumina areslurried in an appropriate vehicle, e.g., water. To incorporatecomponents such as precious metals (e.g., palladium, rhodium, platinum,and/or combinations of the same), stabilizers and/or promoters, suchcomponents may be incorporated in the slurry as a mixture of watersoluble or water-dispersible compounds or complexes. Typically, whenpalladium is desired, the palladium component is utilized in the form ofa compound or complex to achieve dispersion of the component on therefractory metal oxide support, e.g., activated alumina. The term“palladium component” means any compound, complex, or the like which,upon calcination or use thereof, decomposes or otherwise converts to acatalytically active form, usually the metal or the metal oxide.Water-soluble compounds or water-dispersible compounds or complexes ofthe metal component may be used as long as the liquid medium used toimpregnate or deposit the metal component onto the refractory metaloxide support particles does not adversely react with the metal or itscompound or its complex or other components which may be present in thecatalyst composition and is capable of being removed from the metalcomponent by volatilization or decomposition upon heating and/orapplication of a vacuum. In some cases, the completion of removal of theliquid may not take place until the catalyst is placed into use andsubjected to the high temperatures encountered during operation.Generally, both from the point of view of economics and environmentalaspects, aqueous solutions of soluble compounds or complexes of theprecious metals are utilized. For example, suitable compounds arepalladium nitrate or rhodium nitrate.

A suitable method of preparing any layer of the layered catalystcomposite of the invention is to prepare a mixture of a solution of adesired precious metal compound (e.g., palladium compound) and at leastone support, such as a finely divided, high surface area, refractorymetal oxide support, e.g., gamma alumina, which is sufficiently dry toabsorb substantially all of the solution to form a wet solid which latercombined with water to form a coatable slurry. In one or moreembodiments, the slurry is acidic, having, for example, a pH of about 2to less than about 7, or preferably in the range of 3-5. The pH of theslurry may be lowered by the addition of an adequate amount of aninorganic or an organic acid to the slurry. Combinations of both can beused when compatibility of acid and raw materials is considered.Inorganic acids include, but are not limited to, nitric acid. Organicacids include, but are not limited to, acetic, propionic, oxalic,malonic, succinic, glutamic, adipic, maleic, fumaric, phthalic,tartaric, citric acid and the like. Thereafter, if desired,water-soluble or water-dispersible compounds of oxygen storagecomponents, e.g., cerium-zirconium composite, a stabilizer, e.g., bariumacetate, and a promoter, e.g., lanthanum nitrate, may be added to theslurry.

In one embodiment, the slurry is thereafter comminuted to result insubstantially all of the solids having particle sizes of less than about30 microns, i.e., between about 0.1-15 microns, in an average diameter.An exemplary d₉₀ average particle diameter is in the range of about 2.5to about 8 μm. The comminution may be accomplished in a ball mill,circular mill, or other similar equipment, and the solids content of theslurry may be, e.g., about 20-60 wt. %, more particularly about 30-40wt. %.

Additional layers, i.e., the second and third layers may be prepared anddeposited upon the first layer in the same manner as described above fordeposition of the first layer upon the carrier.

Embodiments

Various embodiments are listed below. It will be understood that theembodiments listed below may be combined with all aspects and otherembodiments in accordance with the scope of the invention.

Embodiment 1. An emission treatment system downstream of a gasolinedirect injection engine for treatment of an exhaust stream comprisinghydrocarbons, carbon monoxide, nitrogen oxides, and particulates, theemission treatment system comprising:

-   -   a close-coupled three-way conversion (TWC) composite comprising        a first TWC catalytic material on a flow-through substrate; and    -   an catalyzed particulate filter located downstream of the        close-coupled TWC composite, the catalyzed particulate filter        comprising a second TWC catalytic material that permeates walls        of a particulate filter;    -   wherein the second TWC catalytic material comprises rhodium as        the only platinum group metal.

Embodiment 2. The emission treatment system of embodiment 1, wherein theparticulate filter comprises a mean pore diameter in the range of about13 to about 25 μm.

Embodiment 3. The emission treatment system of any of embodiments 1-2,wherein the particulate filter comprises a wall thickness in the rangeof about 6 mils (152 μm) to about 14 mils (356 μm) and an uncoatedporosity in the range of 55 to 70%.

Embodiment 3.5: The emission treatment system of any of embodiments 1-3,wherein the uncoated porosity is a percentage of volume of pores of theparticulate filter relative to volume of the particulate filter.

Embodiment 4. The emission treatment system of any of embodiments 1-3.5,wherein the catalyzed particulate filter has a coated porosity that isless than an uncoated porosity of the particulate filter.

Embodiment 5. The emission treatment system of any of embodiments 1-4,wherein there is no layering of catalytic material on the surface of thewalls of the particulate filter except optionally in areas of overlappedwashcoat.

Embodiment 5.5. The emission treatment system of any of embodiments 1-5,wherein there is no catalytic material outside pores of the walls of theparticulate filter.

Embodiment 6. The emission treatment system of embodiment 4 or 5 or 5.5,wherein the coated porosity is linearly proportional to a washcoatloading of the TWC catalytic material.

Embodiment 7. The emission treatment system of any of embodiments 4-6,wherein the coated porosity is between 75 and 98% of the uncoatedporosity.

Embodiment 8. The emission treatment system of any of embodiments 4-7,wherein the coated porosity is between 80 and 95% of the uncoatedporosity.

Embodiment 9. The emission treatment system of any of embodiments 1-8,wherein a coated backpressure of the catalyzed particulate filter isnon-detrimental to performance of the engine.

Embodiment 10. The emission treatment system of any of embodiments 1-9,wherein the second TWC catalytic material comprises a d90 averageparticle diameter in the range of about 2.5 to about 8 μm.

Embodiment 11. The emission treatment system of any of embodiments 1-10,wherein the second TWC catalytic material is formed from a singlewashcoat composition that permeates an inlet side, an outlet side, orboth of the particulate filter.

Embodiment 12. The emissions treatment system of any of embodiments1-11, wherein a first single washcoat layer is present on the inlet sidealong up to about 0-100% of the axial length of the particulate filterfrom the upstream end and a second single washcoat layer is present onthe outlet side along up to about 0-100% of the axial length of theparticulate filter from the downstream end, wherein at least one of thefirst and single washcoat layers is present in an amount of >0%.

Embodiment 13. The emissions treatment system of embodiment 12, whereina first single washcoat layer is present on the inlet side along up toabout 50-100% of the axial length of the particulate filter from theupstream end and a second single washcoat layer is present on the outletside along up to about 50-100% of the axial length of the particulatefilter from the downstream end.

Embodiment 14. The emissions treatment system of embodiment 13, whereinthe first single washcoat layer is present on the inlet side along up toabout 50-55% of the axial length of the particulate filter from theupstream end and the second single washcoat layer is present on theoutlet side along up to about 50-55% of the axial length of theparticulate filter from the downstream end.

Embodiment 15. The emissions treatment system of any of embodiments 1-11wherein a single washcoat layer is present on the inlet side along up toabout 100% of the axial length of the particulate filter from theupstream end and there is not a washcoat layer on the outlet side.

Embodiment 16. The emissions treatment system of any of embodiments1-11, wherein a single washcoat layer is present on the outlet sidealong up to about 100% of the axial length of the particulate filterfrom the downstream end and there is not a washcoat layer on the inletside.

Embodiment 17. The emission treatment system of any of embodiments 1-16comprising the second TWC catalytic material in an amount in the rangeof about 0.17 to about 5 g/in³ (about 10 to about 300 g/L).

Embodiment 18. The emission treatment system of any of embodiments 1-17,wherein the second TWC catalytic material consists essentially ofrhodium, ceria or a ceria composite, and alumina.

Embodiment 19. A catalyzed particulate filter located in an emissiontreatment system downstream of a gasoline direct injection engine fortreatment of an exhaust stream comprising hydrocarbons, carbon monoxide,nitrogen oxides, and particulates and downstream of a three-wayconversion (TWC) composite comprising a first TWC catalytic material ona flow-though substrate, the catalyzed particulate filter comprising:

a particulate filter comprising a wall thickness in the range of about 6mils (152 μm) to about 14 mils (356 μm) and a porosity in the range of55 to 70%; and a second three-way conversion (TWC) catalytic material inan amount in the range of about 0.17 to about 5 g/in³ (10 to 300 g/L),the second TWC catalytic material comprising rhodium as the onlyplatinum group metal;

wherein the catalyzed particulate filter has a coated porosity that isless than an uncoated porosity of the particulate filter and a coatedbackpressure that is substantially the same as an uncoated backpressureof the particulate filter.

Embodiment 19.5. The catalyzed particulate filter of embodiment 19,wherein the porosity is a percentage of volume of pores of theparticulate filter relative to volume of the particulate filter.

Embodiment 20. The catalyzed particulate filter of any of embodiments19-19.5, wherein:

-   -   the wall thickness is about 8 mils;    -   the amount of the second three-way conversion (TWC) catalytic        material is in the range of about 0.17 to about 1.5 g/in³ (10 to        90 g/L), the second TWC catalytic material comprising rhodium as        the only platinum group metal; and    -   the particulate filter comprises a mean pore size distribution        in the range of about 13 to about 25 μm.

Embodiment 21. A method of treating an exhaust gas comprisinghydrocarbons, carbon monoxide, nitrogen oxides, and particulates, themethod comprising:

-   -   obtaining a catalyzed particulate filter according to any of        embodiments 1-19.5; and    -   locating the catalyzed particulate filter downstream of a        gasoline direct injection engine and a three-way conversion        (TWC) composite comprising a first TWC catalytic material on a        flow-through substrate;    -   wherein upon operation of the engine, exhaust gas from the        gasoline direct injection engine contacts the catalyzed        particulate filter.

Embodiment 22. A method of making emission treatment system for agasoline direct injection engine, the method comprising:

-   -   positioning a three-way conversion (TWC) composite comprising a        first TWC catalytic material on a flow-through substrate        downstream of the gasoline direct injection engine;    -   obtaining a catalyzed particulate filter comprising a second        three-way conversion (TWC) catalytic material permeating walls        of a particulate filter, the particulate filter comprising a        wall thickness in the range of about 6 mils (152 μm) to about 14        mils (356 μm) and a porosity in the range of 55 to 70% and the        second TWC catalytic material comprising rhodium as the only        platinum group metal;    -   positioning the catalyzed particulate filter downstream of the        TWC composite.

Embodiment 23. The method of embodiment 22, wherein the porosity is apercentage of volume of pores of the particulate filter relative tovolume of the particulate filter.

EXAMPLES

The following non-limiting examples shall serve to illustrate thevarious embodiments of the present invention. In each of the examples,the carrier is a wall-flow cordierite. In each of the examples, porosityis a percentage of volume of pores of the particulate filter relative tovolume of the particulate filter. The examples were made according tothe improved coating technique discussed previously using slurrieshaving a dynamic viscosity in the range of about 5 to less than 40 mPa·sat 20° C.

Example 1

Comparative

A comparative particulate filter of low porosity having a three-wayconversion (TWC) catalyst within the substrate wall was prepared at awashcoat loading of 1 g/in³ (61 g/l). The filter substrate had: an ovalfront face with major axis of 184.9 mm and minor axis of 89.9 mm, anoverall length of 120 mm, 300 CPSI with wall thickness of 8 mil (204μm). The precious metal loading was fixed to 30 g/ft³ with a preciousmetal ratio Pt/Pd/Rh of 0/25/5. The Pd was supported on a ceria-zirconiaoxygen storage component comprising 40% ceria, and the Rh was supportedon an alumina component. The filter substrate had a 48% porosity and amean pore diameter of 13 μm.

Example 2

An inventive particulate filter having a three-way conversion (TWC)catalyst within the substrate wall was prepared at a washcoat loading of1g/in³ (61 g/l). The filter substrate had the same characteristics as inExample 1. The monometallic platinum group metal loading was fixed to 7g/ft³ with a precious metal ratio Pt/Pd/Rh of 0/0/7, thus resulting in acoated filter substrate having only Rh as precious metal. The Rh wassupported on alumina. A ceria-zirconia oxygen storage componentcomprising 40% ceria was also present in the catalyst.

Example 3

The particle filters of Examples 1 and 2 each having 1g/in³ (61 g/l)washcoat were aged at 830° C. bed temperature for 50 hours on engine.The particulate filters were measured under New European Drive Cycle(NEDC) in underfloor position after the same flow-through TWC catalystin close-coupled (CC) position. The close-coupled catalyst was a stateof the art TWC catalyst with an overall precious metal loading of95g/ft³ and Pt/Pd/Rh metal ratio of 0/90/5. The washcoat loading of theTWC catalyst in the close-coupled position was 3.8 g/in³. The TWCcatalyst has been aged at a temperature of 1030° C. for 150 hours onengine. Emissions of total hydrocarbon (HC), carbon monoxide (CO),nitrogenous oxides (NOx) as well as particulate number according to thePMP protocol were measured for the close-coupled and under floorcatalyst and reported in Table 1.

TABLE 1 Example 1 Position Comparative Example 2 Euro 6 Std* HC (g/km)CC 0.0417 0.0360 — CO (g/km) CC 1.4282 1.3565 — NOx (g/km) CC 0.06850.0645 — HC (g/km) Tail Pipe 0.0370 0.0341 0.1 CO (g/km) Tail Pipe1.0391 0.8926 1.0 NOx (g/km) Tail Pipe 0.0219 0.0238 0.068 ParticulateTail Pipe 6.85E+11 5.62E+11 6.00E+11 Number (#/km) HC (%) UF 11.3 5.2 —CO (%) UF 27.2 34.2 — NOx (%) UF 68.0 63.2 — Particulate — 52.3 60.9 —Number Filtra- tion Efficiency (%) *European Commission.

The efficiency conversion of the particulate filter of Example 2 issubstantially the same as that of the comparative particulate filter ofExample 1 with respect to the conversion of gaseous emissions as well asparticle number emissions but at lower platinum group metal loading andcost. To a person skilled in the art, it is evident that the particlefiltration efficiency is not affected by the nature of the platinumgroup metal used in the examples but rather by the filtercharacteristics and amount of washcoat loading. Thus it is of relevancein Example 3 that conversion of gaseous emissions HC, CO and NOx aresubstantially the same between Example 1 and Example 2.

Example 4

Comparative

A comparative particulate filter of high porosity having a three-wayconversion (TWC) catalyst within the substrate wall was prepared at awashcoat loading of 0.83 g/in³ (50 g/l). The filter substrate had: around face with diameter of 143.8 mm and length of 152.4 mm, 300 CPSIwith wall thickness of 8 mil (204 μm). The precious metal loading wasfixed to 3 g/ft³ with a precious metal ratio Pt/Pd/Rh of 0/1/2. The Pdwas supported on a ceria-zirconia oxygen storage component comprising40% ceria, and the Rh was supported on alumina. The filter substrate hada 65% porosity and a mean pore diameter of 20 μm.

Example 5

An inventive particulate filter having a three-way conversion (TWC)catalyst within the substrate wall was prepared at a washcoat loading of1.16 g/in³ (70 g/l). The filter substrate had the same characteristicsas in Example 4. The monometallic platinum group metal loading was fixedto 3 g/ft³ with a precious metal ratio Pt/Pd/Rh of 0/0/3, thus resultingin a coated filter substrate having only Rh as precious metal. The Rhwas supported on an alumina component. A ceria-zirconia oxygen storagecomponent comprising 40% ceria was also present in the catalyst.

Example 6

The particle filters of Examples 4 and 5 were tested as-in in theirfresh state. The particulate filters were measured under New EuropeanDrive Cycle (NEDC) in underfloor position after the same flow-throughTWC catalyst in close-coupled (CC) position. The close-coupled catalystwas the same as used in Example 3. Emissions of total hydrocarbon (HC),carbon monoxide (CO), nitrogenous oxides (NOx) as well as particulatenumber according to the PMP protocol were measured for the close-coupledand under floor catalyst and reported in Table 2.

TABLE 2 Example 4 Position Comparative Example 5 Euro 6 Std* HC (g/km)CC 0.030 0.035 — CO (g/km) CC 1.042 1.226 — NOx (g/km) CC 0.063 0.083 —HC (g/km) Tail Pipe 0.027 0.032 0.1 CO (g/km) Tail Pipe 0.652 0.605 1.0NOx (g/km) Tail Pipe 0.020 0.027 0.068 Particulate Tail Pipe 3.89E+115.2E+11 6.00E+11 Number (#/km) HC (%) UF 10 8.5 — CO (%) UF 41.6 50.7 —NOx (%) UF 68.2 67.4 — Particulate — 72.9 63.8 — Number Filtra- tionEfficiency (%) *European Commission.

The efficiency conversion of the composite is Example 5 is substantiallythe same as that of the state of the art composite of Example 4 withrespect to the conversion of gaseous emissions as well as particlenumber emissions but using only Rh as platinum group metal.

Reference throughout this specification to “one embodiment,” “certainembodiments,” “one or more embodiments” or “an embodiment” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of the phrases such as “in one ormore embodiments,” “in certain embodiments,” “in one embodiment” or “inan embodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The invention has been described with specific reference to theembodiments and modifications thereto described above. Furthermodifications and alterations may occur to others upon reading andunderstanding the specification. It is intended to include all suchmodifications and alterations insofar as they come within the scope ofthe invention.

1. An emission treatment system downstream of a gasoline direct injection engine for treatment of an exhaust stream comprising hydrocarbons, carbon monoxide, nitrogen oxides, and particulates, the emission treatment system comprising: a close-coupled three-way conversion (TWC) composite comprising a first TWC catalytic material on a flow-through substrate; and an catalyzed particulate filter located downstream of the close-coupled TWC composite, the catalyzed particulate filter comprising a second TWC catalytic material that permeates walls of a particulate filter, wherein the second TWC catalytic material comprises rhodium as the only platinum group metal.
 2. The emission treatment system of claim 1, wherein the particulate filter comprises a mean pore diameter of about 13 to about 25 μm.
 3. The emission treatment system of claim 1, wherein: the particulate filter has a wall thickness of about 6 mils (152 μm) to about 14 mils (356 μm) and an uncoated porosity in the range of 55 to 70%, and, the uncoated porosity is a percentage of volume of pores of the particulate filter relative to volume of the particulate filter.
 4. The emission treatment system of claim 1, wherein the catalyzed particulate filter has a coated porosity that is less than an uncoated porosity of the particulate filter.
 5. The emission treatment system of claim 4, wherein there is no layering of catalytic material on the surface of the walls of the particulate filter except optionally in areas of overlapped washcoat.
 6. The emission treatment system of claim 4, wherein the coated porosity is linearly proportional to a washcoat loading of the TWC catalytic material.
 7. The emission treatment system of claim 4, wherein the coated porosity is between 75 and 98% of the uncoated porosity.
 8. The emission treatment system of claim 7, wherein the coated porosity is between 80 and 95% of the uncoated porosity.
 9. The emission treatment system of claim 4, wherein a coated backpressure of the catalyzed particulate filter is non-detrimental to performance of the engine.
 10. The emission treatment system of claim 1, wherein the second TWC catalytic material has a d₉₀ average particle diameter in the range of about 2.5 to about 8 μm.
 11. The emission treatment system of claim 1, wherein the second TWC catalytic material is formed from a single washcoat composition that permeates an inlet side, an outlet side, or both of the particulate filter.
 12. The emissions treatment system of claim 1, wherein a first single washcoat layer is present on the inlet side along up to about 0-100% of the axial length of the particulate filter from the upstream end and a second single washcoat layer is present on the outlet side along up to about 0-100% of the axial length of the particulate filter from the downstream end, wherein at least one of the first and single washcoat layers is present in an amount of >0%.
 13. The emissions treatment system of claim 12, wherein a first single washcoat layer is present on the inlet side along up to about 50-100% of the axial length of the particulate filter from the upstream end and a second single washcoat layer is present on the outlet side along up to about 50-100% of the axial length of the particulate filter from the downstream end.
 14. The emissions treatment system of claim 13, wherein the first single washcoat layer is present on the inlet side along up to about 50-55% of the axial length of the particulate filter from the upstream end and the second single washcoat layer is present on the outlet side along up to about 50-55% of the axial length of the particulate filter from the downstream end.
 15. The emissions treatment system of claim 1, wherein a single washcoat layer is present on the inlet side along up to about 100% of the axial length of the particulate filter from the upstream end and there is not a washcoat layer on the outlet side.
 16. The emissions treatment system of claim 1, wherein a single washcoat layer is present on the outlet side along up to about 100% of the axial length of the particulate filter from the downstream end and there is not a washcoat layer on the inlet side.
 17. The emission treatment system of claim 1, comprising the second TWC catalytic material in an amount of about 0.17 to about 5 g/in³ (about 10 to about 300 g/L).
 18. The emission treatment system of claim 1, wherein the second TWC catalytic material consists essentially of rhodium, ceria or a ceria composite, and alumina.
 19. A catalyzed particulate filter located in an emission treatment system downstream of a gasoline direct injection engine for treatment of an exhaust stream comprising hydrocarbons, carbon monoxide, nitrogen oxides, and particulates and downstream of a three-way conversion (TWC) composite comprising a first TWC catalytic material on a flow-though substrate, the catalyzed particulate filter comprising: a particulate filter having a wall thickness in the range of about 6 mils (152 μm) to about 14 mils (356 μm) and a porosity in the range of 55 to 70%; and a second three-way conversion (TWC) catalytic material in an amount in the range of about 0.17 to about 5 g/in³ (10 to 300 g/L), the second TWC catalytic material comprising rhodium as the only platinum group metal wherein the catalyzed particulate filter has a coated porosity that is less than an uncoated porosity of the particulate filter and a coated backpressure that is substantially the same as an uncoated backpressure of the particulate filter.
 20. The catalyzed particulate filter of claim 19, wherein: the wall thickness is about 8 mils; the amount of the second three-way conversion (TWC) catalytic material is about 0.17 to about 1.5 g/in³ (10 to 90 g/L), the second TWC catalytic material comprising rhodium as the only platinum group metal; and the particulate filter has a mean pore size distribution of about 13 to about 25 μm.
 21. A method of treating an exhaust gas comprising hydrocarbons, carbon monoxide, nitrogen oxides, and particulates, the method comprising: situating the catalyzed particulate filter of claim 1 downstream of a gasoline direct injection engine and a three-way conversion (TWC) composite comprising a first TWC catalytic material on a flow-through substrate, wherein upon operation of the engine, exhaust gas from the gasoline direct injection engine contacts the catalyzed particulate filter.
 22. A method of making emission treatment system for a gasoline direct injection engine, the method comprising: positioning a three-way conversion (TWC) composite comprising a first TWC catalytic material on a flow-through substrate downstream of the gasoline direct injection engine; and positioning the catalyzed particulate filter downstream of the TWC composite, wherein: the obtaining a catalyzed particulate filter comprises a second three-way conversion (TWC) catalytic material permeating walls of a particulate filter, the particulate filter having a wall thickness of about 6 mils (152 μm) to about 14 mils (356 μm) and a porosity of 55 to 70%; and the second TWC catalytic material comprises rhodium as the only platinum group metal. 